Edge Mask Issue - Render nodes
Solution:

Put your shadow element over white background and then multiply the colored layer with it. You get dark edge because you have punched a hole into shadow element with your character and thus the pixel values on transparent edges are darker than they should be. But if you merge over white first, edge color gradient is towards white which in multiply op does not change the brightness.

But overall I think your node setup is a bit confusing, you are using alpha as mix factor while to get shadows balanced you should simply multiply shadow element with color and drive the mix with single value. And if you want more control, use gamma etc color correction operation on shadow element, not blending factor.